A steel sheet for fuel tank is required to fully meet various requirements such as corrosion resistance to the fuel as well as to the surrounding circumference, weldability, and press workability. Of these requirements, corrosion resistance to the fuel is first explained.
In many countries of North, Central and South Americas, Europe, and the like, departure from petroleum dependency is declared as a national energy policy. In such countries, use as alternative motor vehicle fuels such as an alcohol (methanol, ethanol) or the so called gasohol which is a mixture of gasoline with 5 to 20% alcohol is on the increase through these years.
Such alcoholic fuels have corrosivity markedly higher than ordinary gasoline fuels since they tend to (a) contain water; (b) undergo phase separation when water content increases or temperature decrease; and (c) decompose by oxidation to produce an organic acid (for example, methanol is converted into formic acid, and ethanol is converted into acetic acid), leading to the phase separation and formation of the lower layer mainly comprising alcohol and/or the organic acid and water; and (d) alcohol-gasoline mixtures containing more than 40% of methanol do dissolve the steel sheet plated with term metal (Pb--Sn alloy) which is a major current tank material.
In spite of such situation, motor vehicle fuel tanks are required to have no seam welding defects; to undergo no outer or inner surface corrosion; and to generate no floating corrosion product which may result in the blockage of the filter in the fuel circulation system.
Motor vehicle fuel tank materials currently used are, for example, a hot dip Pb--Sn alloy coated steel sheet as disclosed in JP-B 57-61833 and Zn plated steel sheet with thick chromate layer as disclosed in JP-B 53-19981.
The corrosion resistance of such materials to gasoline, alcohol, or alcohol-blended gasoline (hereinafter referred to as "inner surface corrosion resistance"), however, has been quite insufficient. For example, Pb--Sn alloy had a defect that this alloy is highly soluble in methanol and could not be used with the methanol-blended gasoline in actual use.
On the other hand, a Zn electro-plated steel sheet material with an overlying thick chromate layer has some degree of inner surface corrosion resistance due to sacrificial corrosion protection by the Zn. This material, however, has the defect that zinc dissolves at a high rate in an alcohol and gasoline to generate a large amount of floating white precipitate which causes filter blockage in the fuel circulation system, and after the zinc dissolution, the steel substrate starts to corrode to exhibit red rust. This material is also insufficient as a steel sheet for a fuel tank.
When the Zn electro-plated steel sheet is replaced with a steel sheet plated with a zinc-based alloy such as Zn--Ni alloy plated steel sheet disclosed in JP-A 55-110791; Zn--Co alloy plated steel sheet disclosed in JP-B 57-33347; Zn--Fe alloy plated steel sheet disclosed in JP-B 57-61831; Zn--Al alloy plated steel sheet disclosed in JP-B 54-33222; Zn--Ni--Cr alloy plated steel sheet disclosed in JP-A 57-70288; and Zn--Co--Ni alloy plated steel sheet disclosed in JP-B 57-33347, the zinc dissolution rate from the zinc-based alloy plated layer is considerably suppressed compared to the zinc plated layer, and the corrosion of both the outer and the inner surfaces are thereby reduced. These zinc alloy plated steel sheets, however, still suffer from corrosion of the outer surface of the tank and the generation of the floating white precipitate on the tank inner surface which invites filter blockage and corrosion.
In order to obviate the defects as described above, JP-B 2-18981, JP-B 2-18982, and JP-B 3-25349 disclose highly corrosion resistant steel sheets for fuel tank which have excellent inner surface corrosion resistance to the alcohol alone or the alcohol-blended gasoline, and in particular, to the highly corrosive gasoline containing alcohol and formic acid; whose outer surface has excellent corrosion resistance to the surrounding circumstance (hereinafter referred to as "outer surface corrosion resistance"), and which exhibit satisfactory press workability and resistance weldability on manufacturing fuel tank. For example, JP-B 2-18981 describes a steel sheet which has a metal plated layer comprising a Pb/Sn alloy or metals containing Sn as a main component, and an overlying metal powder-containing organic resin film. JP-B 2-18982 and JP-B 3-25349 disclose steel sheets on which a Zn plated layer or plated layer containing Zn as a main component and an overlying metal powder-containing organic resin film are disposed.
The metal powder-containing organic resin films described in these three patent publications are the films in which phenoxy resin constitute 40 to 90% of the organic resin. Therefore, when a steel sheet having such metal powder-containing organic resin film is used in fabricating a gasoline tank, the metal powder may fall off the organic resin film on the outer surface in the course of press working due to insufficiency in the affinity between the hydroxyl group of the phenoxy resin and the metal powder, and as a consequence, plating layers may become peeled off the steel sheet detracting from press workability.
When a steel sheet having such metal powder-containing organic resin film is used for fabricating a gasoline tank, the inner surface of the tank suffers from insufficient inner surface corrosion resistance in the part wherein the steel sheet has experienced the damage of the metal powder falling off or the plated layer peeling off. The undamaged flat region of the tank also suffers from insufficient inner surface corrosion resistance since the corrosive solution is likely to become settled between the resin and the metal powder in the film. Accordingly, it is still difficult to put these steel sheets into actual use.
In addition, in all of the steel sheets in the abovementioned patent publications, the resin film on the surface corresponding to either the outer surface or the inner surface of the tank contains a curing agent as an essential component. When the degree of curing is too high, the organic layer is not readily heat melted, and in the welding, removal of the film will be difficult in the nugget formation process and seamweldability will be deteriorated. More illustratively, even though current passage points are provided by the metal powder, the film remaining unmelted resists the welding, and as a consequence, the nuggets formed do not sufficiently overlap with each other to result in fuel leakage. In some cases, the insufficient welding strength invites peeling. On the other hand, when the film has cured to an insufficient degree and the curing agent which did not react is present in the film, such part suffers from insufficient aggregation and high hydrophilicity of the unreacted reagent to invite invasion of the corrosive elements (acid, chlorine ion etc.) into the film to thereby adversely affect the outer/inner surface corrosion resistance of the tank.
JP-A 64-33173 describes a weldable corrosion resistant epoxy-based coating composition which contains a metal powder mixture of aluminum, stainless steel, and alloys thereof, and a powder consisting essentially of nickel. When this composition is used to coat the steel sheet used for a gasoline tank, affinity between the epoxy resin or the phenoxy resin and the metal powder is insufficient as in the above-described case, and the metal powder is likely to fall off the film during the press working. Therefore, when a tank is coated on both sides with this coating composition, both surfaces of the tank will suffer from the film damage and the associated plating layer damage, and the corrosion resistance is far from being sufficient. The non-damaged flat region on both sides of the tank also suffer from insufficient corrosion resistance since invasion of corrosive ions occurs at the resin/metal powder boundary due to insufficient affinity between the resin and the metal powder.
As described above, the steel sheet materials for fuel tank which have so far been proposed suffer from various insufficiencies in their properties, and the state of the art is that none is in actual use.